Downtown Leesburg is one of those treasures that, until quite recently, was able to maintain itself more or less by default -- a sleepy county seat not much concerned with the outside world because the outside world was not much concerned with it. This began to change dramatically a decade or so ago, as the westward movement of the metropolitan economy picked up speed. Today change is an express train, and little Leesburg has to figure out how to hook up without losing its time-honored sense of self.

Fortunately, the town shows signs of being able to manage the transition. It has expanded aggressively -- a 1984 annexation added nearly 4,000 acres of land that can be developed -- while attempting to protect the irreplaceable character of its historic core. Last week Leesburg made a major move in the right direction by selecting, in an international competition, a design for a new town hall and parking structure in the center of the historic district.

The design challenge here was immense. Architects who entered the competition -- and 201 teams from 35 states and four foreign countries did so -- not only had to face inserting a parking garage into a town whose most distinguished, defining buildings were constructed before automobiles were invented, but also had to decide what, in 1987, constituted an appropriate architectural statement for the town's symbolic center. In addition, they were asked to reserve space, where possible, for small-scale commercial development.

Though it needs refinement in several respects, the winning design by Hanno Weber and Associates of Chicago is basically a subtle, sophisticated response to these conditions. If all goes according to plan, Leesburg will have a new town hall, with a massive conical tower as a symbolic centerpiece, before three years elapse.

The very idea of trying to preserve a cohesive, old-fashioned Main Street center by plunking down a big parking garage in its heart might at first seem a contradiction in terms -- it sounds like one of those nightmarish solutions of urban renewal lore whereby a district is destroyed in order to "save" it. But the added parking really is necessary if downtown Leesburg is to keep an economic edge in competition with burgeoning commercial strip developments and shopping malls, and, happily, there is room for it. The new structure can be fit with relative ease on town-owned land in a key downtown block.

One comes upon central Leesburg as a wonderful surprise. The edges of the old town, particularly along state Rte. 7 on the east and U.S. 15 on the south, are badly frayed by humdrum housing and commercial strip developments, but the center remains pretty much intact. In the small-town southern tradition there's courthouse row, its becolumned brick buildings set back upon green lawns, and there are the eclectic "Main Streets" (here, King and Market), where 18th-, 19th- and early 20th-century buildings coexist along narrow sidewalks with inimitable comeliness. Although there are a few execrable exceptions -- not a whole lot of new building was done in the 1950s, 1960s and 1970s -- the worst damage was done by demolition for parking, producing holes in the town's architectural fabric here and there.

Leesburg's epicenter is the intersection of King and Market streets, an important crossroads dating back more than 200 years. The site for the new buildings faces Market, Wirt and Loudoun streets, with the rear of the King Street stores as a fourth border. Contestants were told to preserve a few buildings on the site (including a row of houses along Wirt Street, the only 18th-century log house left in town and the nondescript Tally Ho Theater), but otherwise were free to locate the new buildings at any point of their choosing. Most of the land now is used as a surface parking lot.

Weber's winning plan places a two-story parking structure at one edge of the site, facing Loudoun Street, and a three-story town hall at the corner of Market and Wirt streets -- a good arrangement in that it plugs two prominent empty spots along these streets. Several entries, by contrast, opted for a space-saving underground lot, but the jurors wisely rejected these schemes (though one earned a third-place award) -- below-grade parking, besides being expensive to build, seems too harsh a contrast to Leesburg's hometown ambiance, especially when there is plenty of room for a structure and ample opportunity to screen it with other buildings.

Attention to such niceties of urban design is, in fact, the paramount characteristic of the winning effort. There's an easy rhythm of open and closed spaces along Market Street, and thoughtful connections were made, for instance, to two existing pathways cutting between buildings on King Street. No effort was spared to make the parking structure a pleasing place -- there are skylights in the roof and attractive, arcaded side walls that can be used by all as a shortcut through the block.

Best of all is the creation of a new "town green" -- a long, linear park framed on one side by the attractive wall of the parking structure (its arcades extending along the side wall of the theater) and on the other by the new town hall and a pleasant row of houses. The town hall is a deft, low-key piece of work except for one important detail -- it confronts Market Street with an end wall punctuated simply by tiny, dielike windows.

A big defect is the architect's failure to screen the Loudoun Street fac ade of the parking structure. As drawn, it's a rather rude gesture to a street that looks to be making a commercial comeback. (Directly opposite there is a nifty new building, by the Leesburg firm of Ballinger and La Rock, that stitches together plain-jane '50s modernism with history-quoting postmodernism of the '80s.) Most likely this is a correctable fault -- if one looks closely, one can see there are more parking spaces in this garage than the 320 asked for, so it should be possible to cut into it enough to make space for a couple of storefronts.

The most unusual, even daring, but not wholly successful aspect of Weber's design, however, is its picturesque signature piece -- a cylindrical tower with a cone-shaped roof that will house the town council chambers and a multipurpose auditorium. It's a strong image that definitely will take some getting used to, this tower, with its abstracted French chateau-esque form (or was the architect thinking of those half-cylinder wings of the original State Capitol in Williamsburg?)

But there is little question that it will be much used. A sculptural centerpiece for the town green, it is set back from Market Street in an alcove separating the new town hall from the Tally Ho; ringed by steps and arcades, it's a linchpin in Weber's excellent pedestrian circulation system. Nor is there much question that it will get built. After the result was made known Wednesday evening, the Leesburg town council voted to appropriate $6.2 million to make sure.

Weber's plan, along with those of the second- and third-place winners and five runners-up, will be on view from 1 to 5 p.m. weekdays in the town council chambers on Loudoun Street, starting Wednesday and ending July 8. Local firms receiving awards were Circa Inc., in association with Keystone Architects, both of Alexandria (third place); and Fabry Associates and Martin & Jones, both of Washington (honorable mentions). The competition was supported by a grant from the National Endowment for the Arts. Jurors included architects Bernard Spring of Boston and Milo Thompson of Minneapolis; urban design consultant Michael Pittas of Los Angeles; planners Mary Means of Alexandria and Martha Mason Semmes of Leesburg; and Leesburg Vice Mayor Charles J. Williams.